1. Field of the invention
The present invention concerns an omnidirectional bar code label scanner comprising a source producing a thin parallel light beam, a dynamic deviator adapted to scan said beam in a pattern centred on a main axis substantially normal to a plane scanning area across which the beam is scanned and through which the label passes, and a photosensor with a light collector aimed at the scanning area to generate a signal corresponding to the bar code that it carries when the pattern passes longitudinally over the label, the deviator comprising scanning optics centred on the main axis and comprising a reflective ring with facets arranged in a regular frustum of a pyramid and a mirror at the center of the ring rotating at constant speed about the main axis on which the beam impinges along said axis so as to scan each facet of the ring consecutively and form at each a reflected beam which traces a diametral segment of the pattern in the scanning area.
2. Description of the prior art
The bar code system is very widely used to identify articles of all kinds in a way that can be read by a machine so that the article codes are identified automatically by scanning a rectangular label carrying a succession of transverse lines that are alternately dark and light and whose width is an integer multiple (usually between 1 and 3) of a unit width. The bar code is read by scanning the label optically in its longitudinal directional at an approximately constant speed to generate an electrical signal corresponding to the code carried by the label.
Scanners moved by hand over the labels have been used to read the bar code; the problem of maintaining a constant scanning speed makes these devices difficult to use effectively without repeating the scanning several times and the response is slow. Automatic scanners are of two main types, "hand" scanners and "swipe" scanners.
Hand scanners comprise a probe with an entry side which is applied to the label in a precise orientation, an unfocussed light source and an optical system which projects an image of the label disposed against the entry window onto a photosensitive strip that is scanned electrically (charge-coupled devices).
Swipe scanners include the type used at the checkouts of supermarkets and the like where the label is passed across the scanner in an approximately transverse direction and a rotating mirror system comprising several differently inclined facets at the center of a faceted reflecting ring forms a pattern comprising several differently oriented parallel arrays. These scanners are relatively tolerant as to the position of the label but relatively intolerant as to its orientation.
Omnidirectional scanners, with which the present invention is concerned, are of the kind defined in the above preamble. The pattern produced by the rotating mirror and the faceted reflective ring comprises the same number of diametral segments as there are facets on the ring, equi-angularly spaced and intersecting at their center on the main axis. This pattern will be referred to where appropriate hereinafter as a radiating pattern.
The segments cross at a point on the main axis in a particular plane (median plane) in the scanning area at a specific distance from the ring. In any plane other than the median plane the segments are tangential to an inscribed circle. The depth of field is determined by the condition that the diameter of the inscribed circle must be small relative to the width of the label.
Effective scanning of the label requires that at least one segment of the radiating pattern passes longitudinally across the label, intersecting all the lines of the bar code carried by the label.
This implies that the angle between two successive diametral segments is less than the acute angle between the diagonals of the rectangle occupied by the lines of the bar code and that the point of intersection of these diagonals is sufficiently close to the center of the pattern. This therefore requires relatively accurate positioning of the label in the scanner.
Incidentally, to obtain a signal with clear transitions readily identifiable by the associated electronics, the diameter of the parallel light beam must be small relative to the unit width of the bar code. The light source is usually a far red semiconductor laser producing a sufficiently small beam cross-section with sufficient energy.
An object of the present invention is to provide an omnidirectional bar code scanner tolerant of significant inaccuracy in the location of the label in and relative to the median plane of the scanning area.